Signal when target is not in HDI

hdinterval.py

@@ -52,7 +52,7 @@ class HDInterval:
 
     def _at(self, target, tolerance, ci=1, jump=1):
         if ci > 1:
-            return 1
+            raise OverflowError()
 
         hdi = self(ci)
         if any(x in tolerance for x in hdi):
@@ -75,4 +75,4 @@ class HDInterval:
                 tolerance *= 10
                 warnings.warn(f'Tolerance reduced: {tolerance}')
 
-        raise OverflowError('Unable to converge')
+        raise FloatingPointError('Unable to converge')